Receptor recognition by viruses is the first and essential step of viral infections. It is an important determinant of viral host ranges and cross-species infections, and a primary target for human intervention. Coronaviruses recognize a variety of receptors, infect many hosts, and are significant health threats to humans and other animals. NL63 coronavirus (NL63-CoV), a prevalent human respiratory virus, is the only group-I coronavirus known to use angiotensin-converting enzyme 2 (ACE2) as its receptor, whereas other group-I coronaviruses use aminopeptidase-N (APN). Curiously, ACE2 is also used by group-II SARS coronavirus (SARS-CoV), the agent for the fatal human severe acute respiratory syndrome (SARS). Defined receptor-binding domains (RBDs) on the spike proteins of NL63-CoV and SARS-CoV bind ACE2 with high affinity. This research investigates the receptor recognition mechanisms and cross-species infections of coronaviruses. Our preliminary studies have determined the crystal structures of NL63-CoV RBD complexed with human ACE2 and of SARS-CoV RBD complexed with human ACE2. NL63-CoV and SARS-CoV RBDs have no structural homology in cores or receptor-binding motifs (RBMs) that directly contact ACE2, but recognize the same virus-binding hotspot on ACE2. Among group-I coronaviruses, RBD cores are conserved, but RBMs are variable, explaining how these viruses recognize different receptors. We have also determined crystal structures of the RBDs from various SARS-CoV strains complexed with ACE2 proteins from humans and palm civets, revealing mechanisms whereby SARS-CoV transmitted between the two hosts. There are three specific aims in this project. Aim 1 focuses on the receptor recognition mechanisms of group-I NL63-CoV. It investigates specific functions of the virus-binding hotspot on human ACE2 in NL63-CoV binding, and examines interactions between the NL63-CoV spike protein and ACE2 proteins from non-human hosts. Aim 2 focuses on the receptor recognition mechanisms of other group-I coronaviruses. It investigates specific functions of the RBMs of other group-I coronavirus spike proteins, and studies whether modifying the RBMs changes the receptor specificities of the spike proteins. Aim 3 focuses on the receptor recognition mechanisms of group-II SARS-CoV. It investigates why mutations in the RBM region of past SARS-CoV strains were selected, whether and how these mutations affect receptor binding, and what combinations of these mutations may appear in potential future SARS-CoV strains. The research approaches of this proposal include crystallographic analysis of the virus-receptor interfaces, identification of structural elements important for virus-receptor interactions, and characterization of these elements using crystallographic, biochemical, molecular, and virological tools. Overall, these studies will provide the molecular and structural basis for understanding viral evolution, virus-receptor interactions, viral host ranges and cross-species infections. They will also guide the development of novel antiviral strategies against coronavirus infections.